The present invention relates to a process for manufacturing coarse, crystalline alumina which, after a generally known method of further preparation is advantageously suitable for use as a grinding, lapping and polishing material or for refractory purposes.
In the abrasive manufacturing industry there is a great demand for single crystal corundum of particle size 800 to 200 on the FEPA scale, corresponding to an average particle size of about 10-70 .mu..
It has already been pointed out several times that it is basically possible to manufacture the desired single crystals of corundum directly in the production of alumina, thus circumventing the expensive and technically involved route employing melting and sintering processes. Up to now, however, all economically acceptable efforts have failed because of the inadequate size of the crystals obtained and/or because of their extremely thin platelet appearance.
The present invention overcomes these difficulties. The alumina according to the invention comprises thick platelet corundum crystals which are delimited by natural crystal faces, occur in the above mentioned particulate range and can be manufactured economically.
Alumina is normally produced on a large scale using the Bayer process in which bauxite is digested in a caustic soda solution in which aluminum hydroxide is precipitated in the form of agglomerates up to approximately 100.mu. in size. The aluminum hydroxide is converted to alumina by calcination in rotary or fluidized bed furnaces.
Alumina produced this way is not, or only to a very limited degree, useable for the above mentioned purposes, possibly for example as a polishing material.
There has been no lack of attempts to produce via the Bayer process alumina products which exhibit in particular the properties of grinding and lapping abrasive materials. It is known that the addition of small amounts of so-called calcining agents or mineralisers accelerate the transformation to .alpha.-aluminum oxide and/or lower the temperature at which the transformation takes place. At the same time there is a shift in the particle size distribution towards larger size particles.
Effective in this respect are halides, either singly or in combination, in particular the fluorides NaF, CaF.sub.2, AlF.sub.3 and Na.sub.3 AlF.sub.6, compounds of boron, vanadium and phosphorus (see e.g. German patents or patent application Nos. DE-AS 10 41 853, DE-AS 11 59 418, DE-AS 17 67 511, DE-OS 26 23 482). According to DE-AS 11 59 418 some tenths of a percent of gaseous hydrogen fluoride in the atmosphere of the kiln or furnace has the same effect.
The temperature of the conversion to .alpha.-aluminum hydroxide and its particle size can be varied within certain limits depending on the rate of throughput, rate of heating up and the type and amount of fluorine compound.
According to DE-AS No. 28 50 064 multiple crystallization accompanied by the use of a seeding agent, an already calcined product, fluorine containing additives and aluminum hydroxide agglomerates of &gt;60 .mu. corundum crystals, it is possible to manufacture .alpha.-Al.sub.2 O.sub.3 with diameters D as the largest dimension perpendicular to the c-axis of a size 16.mu. to at most 250.mu. with diameter to height ratios D/H of 3-7, the height H being the largest dimension parallel to the c-axis.
With processes employing no seeding crystals only .alpha.-Al.sub.2 O.sub.3 crystals of extremely thin platelet, hexagonal appearance have been obtained up to now. The crystals have at best a diameter D of 25.mu., the greater part however of diameter around 10.mu.. The height H in some samples examined lie at about one quarter of the diameter D; in over 80% of the crystals the height H is only one sixth to one tenth of the diameter D.
The disadvantage of such crystals for grinding, lapping and polishing purposes lies in the too small particle size and/ or in particular in the large diameter to height ratio D/H. Plate-shaped corundum crystals less than 10.mu. in diameter are of hardly any use to the surface preparation industry. Crystals of large diameter but large D/H ratios fracture very easily when used as a grinding agent, especially when lapping and polishing, and produce cutting edges of random geometry. The claimed advantage of the single crystal particles, with constant cutting geometry on all particles due to the natural crystal faces and the high, specific proportion of cutting edges is at least partly eliminated by such fracturing of particles.
For this reason alumina products made via the calcination of aluminum hydroxide have up to now not found the expected acceptance in surface treatment applications.